actinomycosis (lumpy jaw)
  Inflammatory disease of cattle, hogs, and sometimes humans, caused by
  actinomyces; causes lumpy tumors of the mouth, neck, chest, and abdomen.

Addison’s disease
  Caused by partial or total failure of adrenocortical function;
  characterized by a bronze-like skin color and mucous membranes, anemia,
  weakness, and low blood pressure.

ad libitum
  At the discretion of the performer.  Giving license to alter or omit a
  part.

affusion
  Pouring on of liquid, as in baptism.

ague
  Alternating periods of chills, fever, and sweating.  Used in reference to
  the fevers associated with malaria.

aletris farinosa (Colicroot, star grass, blackroot,
blazing star, and unicorn root )
  Bitter American herb of the Bloodwort family, with small yellow or white
  flowers in a long spike (Aletris farinosa and A. aurea).

algid
  Cold; chilly.

alkanet
  European perennial herb (Alkanna tinctoria) having cymes of blue flowers
  and red roots.  The red dye extracted from the root.  Plants of the
  Eurasian genus Anchusa, having blue or violet flowers grouped on
  elongated cymes.

allyl
  Univalent, unsaturated organic radical C3H5.

aloin
  Bitter, yellow crystalline compound from aloe, used as a laxative.
alum
  Double sulfates of a trivalent metal such as aluminum, chromium, or iron
  and a univalent metal such as potassium or sodium, especially aluminum
  potassium sulfate, AlK(so4)2 12H2O, widely used in industry as
  clarifiers, hardeners, and purifiers and medicinally as topical
  astringents and styptics.

anemonin
  Acrid poisonous compound containing two lactone groups; obtained from
  plants of the genus Anemone and genus Ranunculus, containing the
  buttercups.

aneurysm (aneurism)
  Localized, blood-filled dilatation of a blood vessel caused by disease
  or weakening of the vessel wall.

animadversion
  Strong criticism.  Critical or censorious remark: 

anise
  Aromatic Mediterranean herb (Pimpinella anisum) in the parsley family,
  cultivated for its seed-like fruits and the oil; used to flavor foods,
  liqueurs, and candies.

anodyne
  Relieves pain.

antipyrine (antipyrin, phenazone)
  Analgesic and antipyretic (reduces fever) C11H12N2O formerly used, but
  now largely replaced by less toxic drugs such as aspirin.

antrum
  Cavity or chamber, especially in a bone.  Sinus in the bones of the upper
  jaw, opening into the nasal cavity.

apomorphine
  Poisonous white crystalline alkaloid, C17H17no2, derived from morphine
  and used to induce vomiting.

arnica
  Perennial herbs of the genus Arnica.  Tincture of the dried flower heads
  of the European species A. montana, applied externally to relieve the
  pain and inflammation of bruises and sprains.